REACTIONS TO LIGHT IN CILIATKS AND FLAGELLATES. 



37 



orientation theory. This observation was made repeatedly in a very 

 large number of cases ; not a single exception to it was observed. The 

 swinging of the anterior end is continued past the point where the light 

 falls directly upon it until the animal is directed again into the shadow, 

 as illustrated in the diagram (Fig. 12, , 1,2, 3, 4). 



2. The Stentor may reach the line with the aboral side directed away 

 from the source of light. In this case it turns (as usual) toward the 

 aboral side, thus swinging its anterior end aivay from the source of light. 



3. The Stentor may reach the line with the aboral side directed 

 upward or downward or in some intermediate position. In every case 

 it turns toward the right aboral side, in whichever way this is directed. 



x 



a 



s 



A 



B 



FIG. 13 * 



The writer wishes it to be understood that the foregoing statements 

 as to the direction in which the animal turns are presented, not merely 

 as interpretations in accordance with a certain theory, but as direct, 

 unequivocal observations, many times repeated. Thus, on passing from 

 a darker to a lighter area, even when the light comes from one side. 

 the Stentors react merely to the difference in illumination, without 

 regard to the direction from which the light comes. The direction of 

 turning is determined throughout by an internal factor, not by the side 

 of the animal on which the light falls, nor by the direction of the rays of 

 light. We have put the theory of orientation by direct differential 



*FiG. 13. Another method of testing the manner in which Stentor reacts to 

 light. For a side view of this apparatus, see Fig. 14. Light comes from the 

 left side, in the direction indicated by the arrows. A screen (s) is interposed 

 between the source of light and the vessel containing the Stentors. This screen 

 is of such a height (as illustrated in Fig. 14) that it cuts off the light from the 

 half (A) of the vessel next to the window, leaving the other half (B) lighted. 

 At c (i, 2, 3, 4, 5) is seen the reaction method of a specimen which swims across 

 the line x-y, separating the shaded half A from the lighted half #. 



